solar plexus Plexus (from the Latin for ‘braid’) is the name used to describe a network of nerves; a ‘spaghetti junction’ which many nerves pass through and where their branches split off to merge with those of others. A plexus such as this one, where most of the nerves belong to the autonomic nervous system, also embeds relay stations — the ganglia where sympathetic nerve fibres coming from the spinal cord relay at synapses with the post-ganglionic nerves that then proceed to their destination on smooth muscles or glands.

Solar is the more colourful adjective for the plexus, otherwise known as coeliac (derived from the Greek for ‘belly’). Its widely-radiating incoming and outgoing nerves, linking the two coeliac ganglia, are fancifully likened to the sun's rays. It lies mainly on the front of the aorta, where this main artery enters the abdomen by passing down through the diaphragm — at the ‘pit of the stomach’, and behind the stomach itself. The sympathetic nerves to the abdominal organs, glands, and blood vessels relay here, or pass through in branches to other satellite plexuses and relay there. Since these nerves originate from the lower thoracic segments of the spinal cord, they reach the plexus by passing down from the thoracic cavity. The adrenal glands are among the organs innervated via the plexus, which therefore incorporates the nerve pathway for switching on the release of adrenaline into the bloodstream from the adrenal medulla. In addition to the sympathetic components, branches from the vagus nerves also pass this way, to be distributed to the abdominal organs. They carry parasympathetic nerve fibres descending from the brain stem, which activate the muscle and glands of the gut, and ascending fibres serving visceral sensation. Injections that block nerve transmission in the plexus may be helpful in the treatment of intractable abdominal pain, such as in cancer of the pancreas. A coeliac plexus reflex is described, consisting of a fall in blood pressure when the upper abdominal organs are handled during a surgical operation. A reflex as well as a mechanical effect may also be involved in the sensation of being ‘winded’ by a blow in this region.

Sheila Jennett

so·lar plex·us / ˈpleksəs/ • n. a complex of ganglia and radiating nerves of the sympathetic system at the pit of the stomach. ∎  the area of the body near the base of the sternum: she felt as if someone had punched her in the solar plexus.

solar plexus (coeliac plexus) (soh-ler pleks-ŭs) n. a network of sympathetic nerves and ganglia high in the back of the abdomen.